The present invention relates mainly to powder metallurgy processing, more particularly to manufacturing of powders of various inorganic compounds by so called combustion synthesis or self-propagating high temperature synthesis. This method is well known and has been successfully employed for preparation of powders from ceramic and non ceramic compounds, e.g. cermets, composites, intermetallic compounds, complex salts, etc. The present invention refers primarily to a dedicated apparatus suitable for carrying out of such self-propagating high temperature synthesis.
There are known numerous publications describing self-propagating high temperature synthesis. This process is based on inducing a high temperature self-sustaining combustion reaction within a mixture of initial components, required for synthesis of a compound, providing that upon mixing these components forms an exothermic system. By virtue of very high temperatures, accompanying the combustion reaction the compound is synthesized.
The method has been successfully employed for manufacturing of many compounds, including TiC, WC/TiC, TiC+Alumina, Ti5Si3, NiAl, TiN, Fe2O3, LiNbO3.
There are known in the art various apparatuses designed for carrying out self-propagating high temperature synthesis. For the sake of brevity, these apparatuses will be referred-to further as SHS reactors and the combustion reaction will be referred-to as SHS reaction.
Most of the known in the art SHS reactors are designed to operate periodically. Typical examples of such reactors are disclosed in U.S. Pat. No. 5,178,844 or RU874165. The reactors disclosed in the above references comprise hermetically closed reaction vessel, provided with a cooling jacket. The SHS reactor disclosed in U.S. Pat. No. 5,178,844 comprises a reactor chamber divided into a combustion zone and a reaction zone.
A combustion mixture is injected into the combustion zone and is combusted therein. The reactor described in this reference is suitable for producing of silicon nitride.
Usually after the reaction chamber is filled with the charge mixture of initial components the reactor is vacuumized and the mixture is ignited to initiate the SHS process. Upon completing the process the reactor is cooled, the gases that were produced during the synthesis are evacuated therefrom and the charge in the form of smelted lumps is taken away. The charge is then comminuted to obtain the necessary powder size and powder size distribution.
Unfortunately, there exist fundamental disadvantages associated with the SHS reactors operating periodically. These reactors have relatively low capacity due to the necessity to periodically terminate the process for recharging the reactor.
Furthermore, the final product is not homogeneous due to significant temperature gradients accompanying the synthesis. Still further disadvantage is associated with the necessity in additional equipment and energy consumption required for comminuting.
The present invention, which will be described, further refers to a SHS reactors operating in a continuation mode.
The SHS reactors operating continuously are also known. An example of such SHS reactor is described in RU1770274. This reactor is equipped with a feeder provided with a rotating worm adapted to feed the charge from the feeder into combustion zone. For disintegrating of the smelted lump a dedicated rotating cutting disc is employed, which is mounted co-axially with the reactor exit.
Unfortunately, despite of substantial advantage achieved due to continuous mode of operation the construction of this reactor is associated with some substantial shortcomings. The combustion front is not prevented from expanding beyond the combustion zone up to the worm and feeder and this event can bring the reactor completely out of order or even destroy it. The reactor is not equipped with a means for preliminary heating of the charge and thus it cannot process low exothermic mixtures. The reactor is not provided with a means for advancing the smelted charge through the combustion zone.
The more advanced construction of a SHS reactor operating continuously is mentioned in WO90/11857 and RU2054376. This reactor comprises main rotor or drum adapted to receive the flow of reaction components, which are required for the synthesis of a compound. The flow fed from a feeder approaches the drum""s periphery, deposits thereon in the form of a layer and is preheated thereon by a heat supplied to the main drum. Since the drum rotates the layer is continuously forwarded by drum""s periphery toward the combustion zone where it is ignited.
The angular velocity of rotation is kept to ensure, that the combustion front propagates opposite to the flow. After the layer has passed the combustion zone and the compound is synthesized it is comminuted between the main drum and auxiliary roll, which is resiliently pressed to the main drum.
This reactor has been successfully employed for manufacturing of titanium carbide powder and titanium-tungsten carbide powder as disclosed in RU2054377, RU2069175. Nevertheless, the construction of this reactor is also associated with significant shortcomings.
The main shortcoming of the above reactor is its insufficient safety, since the reactor is not equipped with a means for monitoring the current position of the combustion front and detecting the event when the combustion front expands towards the feeder, for example due to breakdown of the drum drive or due to electricity supply breakdown. Besides, since the drum cooling system is not provided with a temperature control the charge within the feeder is not prevented from inflammation.
Furthermore, since the axes of rotation of the main and auxiliary drum lay within the same horizontal plane the charge flow can slope down from the drum""s periphery. This may be associated with insufficient reliability of operation due to possibility for discontinuation of the flow and terminating of the self-sustaining combustion.
The final product synthesized in this reactor is not pure, since the charge is not prevented from entering into receptacle for collecting the ready product and therefore it can be contaminated.
The capacity of this reactor is relatively low, since it is not equipped with a mechanism for densification of the charge and thus the charge can be easily expelled from the combustion zone and lost.
The assortment of compounds, which can be synthesized in this reactor is limited, since it is not suitable neither for processing of low exothermic charges and compounds requiring to use reactant gas, e.g. Nitrogen, Oxygen etc., nor for processing under vacuum.
In conclusion it should be emphasized that despite the fact that different SHS reactors have been devised there is still exists a need to provide a new and improved reactor, which will enable carrying out the SHS synthesis safely, efficiently and economically.
The object of the present invention is to provide a new and improved SHS reactor, operating continuously and enabling sufficiently reduce or overcome the above-mentioned drawbacks of the known in the art SHS reactors.
In particular, the main object of the present invention is to provide a new and improved SHS reactor in which the combustion front is prevented from expanding beyond the combustion zone and thus the combustion synthesis can be carried out safely.
The further object of the present invention is to provide a new and inexpensive SHS reactor in which the charge flow is continuous and does not disengage the combustion front and thus the combustion synthesis is carried out reliably.
The third object of the present invention is to provide a new and improved SHS reactor, in which the final product is not contaminated.
Still further object of the invention is to provide a new and versatile SHS reactor, which operates automatically and which is capable to process various charges either in the presence of gases or under vacuum.
The above and other objects and advantages of the SHS reactor of the present invention can be achieved in accordance with the following combination of its essential features, referring to different embodiments thereof.
According to one of the preferred embodiments, the apparatus of the invention comprises the following main components
i) a feeding receptacle for storing the charge therein, said feeding receptacle being in communication with
ii) a closed reactor chamber, configured as hot-insulated cylindrical housing provided with
an inlet opening for entering the charge mixture thereinto,
a combustion zone provided with igniting means for igniting the charge Mixture to establish self-sustaining combustion reaction between the ingredients,
an outlet opening for discharging of the synthesized compound from the housing,
xe2x80x83said reactor chamber is provided with a forwarding means, residing within the housing and driven by a drive, said forwarding means is capable to advance continuously the charge mixture towards the combustion zone and to draw the charge mixture therethrough, said forwarding means is located within the housing in such a manner, that the charge mixture deposits thereon substantially as a layer displaceable by the forwarding means,
iii) a monitoring means capable to monitor the current location of the combustion front established during said combustion reaction and to detect when said combustion front expands beyond the combustion zone,
iv) an extinguishing means capable to terminate the combustion reaction substantially automatically when the combustion front expands beyond the combustion zone,
v) a retaining means capable to prevent the said layer from separating from the cylindrical periphery surface of the main drum,
vi) a comminuting means capable to disintegrate the smelted layer after it has passed the combustion zone and to convert it into particulate synthesized compound,
vii) an instrumentation and control means required for running the apparatus.
In the further embodiments, said apparatus is provided with various additional components, enabling achieving the above objects of the invention.
Among these components can be mentioned a compacting means capable to compact the said layer before it enters the combustion zone, a scraping means for separation of the smelted layer from the forwarding means, a heating means for heating at least part of the forwarding means, before the charge resides thereonto, a heating means for heating the charge before it enters the housing, a gas supply means for supplying of at least one gas reactant as required for synthesizing the compound and with a means for evacuation and filtering of the residual gas products generated during the combustion reaction, a blowing means for cleaning the interior of the reactor chamber by blowing therethrough of a cleaning gas, a vacuum means for vacuumization the reactor chamber, at least one window for visual observation of the chamber interior, a de-contamination means for preventing contamination of the synthesized compound by the charge mixture, which did not undergo the combustion reaction.
The present invention in its various embodiments has only been summarized briefly. For better understanding of the present invention as well of its advantages, reference will now be made to the following description of its embodiments with reference to the accompanying drawings.